[3rdparty] Bump symfony/event-dispatcher to 2.7.7

diff --git a/3rdparty b/3rdparty
index 6c70738949fbb2add7850b1f55fb7f10524c68e9..3215390b01ab3e76f15825ff126961bbb5a86328 160000 (submodule)
--- a/3rdparty
+++ b/3rdparty
@@ -1 +1 @@
-Subproject commit 6c70738949fbb2add7850b1f55fb7f10524c68e9
+Subproject commit 3215390b01ab3e76f15825ff126961bbb5a86328